Elastic fiber degradation and intraalveolar fibrosis, with invasion of air spaces by connective tissue cells and their extracellular products of protein synthesis, were considered of critical importance in the pathogenesis of the remodeling of lung tissue that characterizes the pulmonary fibrosis in Langerhans cell granulomatosis. Connective tissue cells migrate into the air spaces through breaks in the alveolar epithelial basement membranes; proliferation of these cells and synthesis of connective tissue proteins leads to alveolar collapse and obliteration of alveolar lumina. Elastic fiber degradation leads to eventual disappearance of old alveolar boundaries, and further collagen deposition completes the remodeling process.